Carbohydrates stability

Lowendahl, L., and Samuelson, O. (1977). Carbohydrate stabilization during kraft cooking with addition of anthraquinone. Sven. Papperstidn. 80, 549-551. 

Minor, J. L. Sanyer, N. Carbohydrate Stabilization with Iodide in Oxygen... 

FIGURE 10.13 Redox reactions of AHQ ion radicals and dianions that lead to lignin fragmentation and carbohydrate stabilization. (Dimmel, D.R., J Wood Chem Technol, 5, 1, 1985.)... 

Ando S, et al. (1999). PLGA microspheres containing plasmid DNA Preservation of supercoiled DNA via cryopreparation and carbohydrate stabilization. J. Pharm. Sci. 88 126-130. 

The majority of easily detected compounds at solid anodes under constant applied potentials are self-stabUized via tt-resonance. Therefore, a desirable characteristic of electrodes in dc amperometry is inert. The electrode serves as a sink to provide and remove electrons with no direct involvement in the reaction mechanism. Since TT-resonance does not exist in polar ahphatic compounds (e.g., carbohydrates), stabilization of reaction intermediates is actively achieved via adsorption at clean noble metal electrodes. Faradaic processes that benefit from electrode surface interactions are described as electrocatalytic. Unfortunately, an undesirable consequence of this apiproach is the accumulation of adsorbed carbonaceous materials, which eventually foul the electrode surface. 

The most stable protected alcohol derivatives are the methyl ethers. These are often employed in carbohydrate chemistry and can be made with dimethyl sulfate in the presence of aqueous sodium or barium hydroxides in DMF or DMSO. Simple ethers may be cleaved by treatment with BCI3 or BBr, but generally methyl ethers are too stable to be used for routine protection of alcohols. They are more useful as volatile derivatives in gas-chromatographic and mass-spectrometric analyses. So the most labile (trimethylsilyl ether) and the most stable (methyl ether) alcohol derivatives are useful in analysis, but in synthesis they can be used only in exceptional cases. In synthesis, easily accessible intermediates of medium stability are most helpful. 

It IS not possible to tell by inspection whether the a or p pyranose form of a par ticular carbohydrate predominates at equilibrium As just described the p pyranose form IS the major species present m an aqueous solution of d glucose whereas the a pyranose form predominates m a solution of d mannose (Problem 25 8) The relative abundance of a and p pyranose forms m solution depends on two factors The first is solvation of the anomeric hydroxyl group An equatorial OH is less crowded and better solvated by water than an axial one This effect stabilizes the p pyranose form m aqueous solution The other factor called the anomeric effect, involves an electronic interaction between the nng oxygen and the anomeric substituent and preferentially stabilizes the axial OH of the a pyranose form Because the two effects operate m different directions but are com parable m magnitude m aqueous solution the a pyranose form is more abundant for some carbohydrates and the p pyranose form for others... 

A point to be emphasized about glycoside formation is that despite the presence of a number of other hydroxyl groups m the carbohydrate only the anomenc hydroxyl group IS replaced This is because a carbocation at the anomenc position is stabilized by the nng oxygen and is the only one capable of being formed under the reaction conditions... 

Section 28 8 The most common form of DNA is B DNA which exists as a right handed double helix The carbohydrate-phosphate backbone lies on the outside the punne and pyrimidine bases on the inside The double helix IS stabilized by complementary hydrogen bonding (base pairing) between adenine (A) and thymine (T) and guanine (G) and cytosine (C)... 

Similar materials are available based on potato starch, eg, PaseUi SA2 which claims DE below 3 and has unique properties based on its amylose—amylopectin ratio pecuhar to potato starch. The product contains only 0.1% proteia and 0.06% fat which helps stabilize dried food mixes compounded with it. Another carbohydrate raw material is waxy-maize starch. Maltodextrias of differeat DE values of 6, 10, and 15, usiag waxy-maize starch, are available (Staley Co.). This product, called Stellar, is offered ia several physical forms such as agglomerates and hoUow spheres, and is prepared by acid modification (49). Maltodextrias based oa com starch are offered with DEs of 5, 10, 15, and 18 as powders or agglomerates (Grain Processing Corp.). 

N HCl, EtOH, reflux, 90% yield for cholesterol. Although a direct stability comparison was not made, this group should be more stable than the THP group for the same reasons that the anomeric ethers of carbohydrates are more stable than their 2-deoxy counterparts. 

The prevalence of diols in synthetic planning and in natural sources (e.g., in carbohydrates and nucleosides) has led to the development of a number of protective groups of valuing stability to a substantial array of reagents. Dioxolanes and diox-anes are the most common protective groups for diols. The ease of formation follows the order ... 

Characteristics due to chemical functionalities (e.g., carboxyl groups) of sample components that control solubility of the sample in aqueous media, viscosity of carbohydrate/polysaccharide solutions, and stability of obtained solutions. 

When this sequence is applied to carbohydrates, the first step takes place intramoleai-larly to yield a cyclic hemiacetal. The second step is intermolecular, requires an alcohol R"OH as a reactant, and proceeds readily only in the presence of an acid catalyst. An oxygen-stabilized carbocation is an intermediate. 

In carbohydrates in which benzyl groups are used extensively for protection, the stability of the benzyl groups toward electrophilic reagents is increased by the presence of electron-withdrawing groups in the ring." ... 

The diastereodifferentiating effect of the galactosylamine template in these Strecker reactions is rationalized in terms of a preferred conformation 5 of the Schiff bases which is stabilized by a (7i-double bond into the carbohydrate ring. This conformation is supported by a strong NOE in the H-NMR spectrum between the anomeric and the iminc proton. 

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